Digital Fluoroscopy- Part III

Digital Fluoroscopy - Part III




The next improvement that will be seen will be flat panel imagers composed of silicon pixel detectors (SPD). These are direct detectors. Eventually all television camera devices will be obsolete.

Video System


The video system with conventional fluoroscopy uses a 525-line system.
Such a system is adequate for DF, but the higher spatial resolution can be obtained with a 1000-line system.




Interlaced Versus Progressive Mode


The method used for conventional fluoroscopy is the interlaced mode, where two fields of 2621/2 lines were read in 1/60th s (17 ms) to form a 525-line video frame in 1/30th s (33 ms).
In DF, the camera tube reads in progressive mode.
When reading the video signal in the progressive mode, the electron beam of the television camera tube sweeps the target assembly continuously from top to bottom in 33 ms.
What will occur is two interlaced fields with be combined to create the full image.
The video image is similar to form on the television monitor. There is no interlace of one field with another. This produces a sharper image with less flicker on the image screen.


Signal-to-Noise Ratio (SNR)


All analog electronic devices are inherently noisy.
Because of heated filaments and voltage differences, there is always a very small electric current flowing in any circuit. This is called background electronic noise. This electronic noise obscures the electronic signal. Because conventional television camera tubes have an SNR of about 200:1, the maximum output signal will be 200 times greater than the background electronic noise.
A SNR of 5:1 is minimally visible. A SNR og 200:1 is not sufficient for DF because the video signal is rarely at maximum and lower signals become even more lost in the noise.
This is very true for subtraction techniques. Image contrast resolution is degraded a lot by a system with a low SNR. When comparing the dynamic ranges where the SNR is 200:1, this is around 2^8 and the 1000:1 is 2^10.
The second set of number units are much more compatible with the computer systems of today.




Computer


Minicomputers and microprocessors are used with DF. The capacity of the computer is an important factor in determining image quality, the manner and speed of image acquisition, and image processing and manipulation.


Important characteristics of a DF system are:
Image matrix size
The system dynamic range
Image acquisition rate
The output signal from the television camera tube is transmitted by cable to an analog-to-digital converter (ADC).
The ADC accepts the continuously varying television camera output signal, the analog signal, and digitizes it.



To be compatible with the computer, the ADC must have the same dynamic range as the DF system. An 8-bit ADC would convert the analog signal into values between 0 and 255.
A 10-BIT ADC would be more precise, with an analog-to-digital conversion range from 0 to 2^10 or 0 to 1023. The output of the ADC is then transferred to main memory and manipulated so that a digital image in matrix form is stored. The dynamic range of each pixel, the number of pixels, and the method of storage determine the speed with which the image can be acquired, processed, and transferred to an output device.
The output of the ADC is then transferred to the main memory and manipulated so that a digital image in matrix form is stored. The dynamic range of each pixel, the number of pixels, and the method of storage determine the speed with which the image can be acquired, processed, and transferred to an output device.

If the image storage is in primary memory, which is usually the case, then data acquisition and transfer can be as rapid as 30 images per second. In general, if the image matrix is doubled, (ex: 512 to 1024), the image acquisition rate will be reduced by a factor of four.
A representative system may be able to acquire 30 images per second in a 512 x 512 matrix mode. If a higher spatial resolution image is required and a 1024 x 1024 mode is requested, then only 8 images per second can be acquired. This is limited by the time required to conduct and transfer the amount of data from one segment of memory to another.




Image Formation


The principle advantage of DF exams are the image subtraction techniques that can be performed and the ability to visualize vascular markings with a venous injection of contrast material. A disadvantage is an area beam must be used to perform this, which reduces image contrast because of the scattered radiation. Image contrast will then have to be enhanced electronically. Image contrast is improved by subtraction techniques that provide instantaneous viewing of the subtracted image, during the passage of a bolus of contrast medium.
Digital fluoroscopy provides better contrast resolution by postprocessing image subtraction.


Temporal subtraction and energy subtraction are two methods that are used in DF.
Each have their own advantages and disadvantages.


Temporal Subtraction:


A single kVp setting is used;

Normal x-ray beam filtration is adequate;
Contrast resolution of 1 mm at 1% is achieved;
Simple arithmetic image subtraction is necessary;
Motion artifacts are a problem;
Total subtraction of common structures is achieved;
Subtraction possibilities are limited by number of images.



Energy Subtraction:


Rapid voltage switching is required;
X-ray beam filter switching is preferred;
Higher x-ray intensity is required for comparable contrast resolution;
Complex image subtraction is necessary;
Motion artifacts are greatly reduced;
Some residual bone may survive subtraction;
Many more types of subtraction images are possible.


Temporal Subtraction techniques are more frequently used because of the high-voltage generator limitations in the energy subtraction mode. When the two techniques are combined, the process is called hybrid subtraction. Image contrast is enhanced still further by hybrid subtraction because of reduced patient motion between subtracted images.



Temporal Subtraction


Temporal subtraction refers to a number of computer-assisted techniques whereby an image is obtained at one time is subtracted from an image obtained at a later time. If, during the intervening period, contrast material was introduced into the vessels, the subtracted image will only contain vessels filled with the contrast media. There are two methods that are commonly used: mask mode and time-interval difference mode (TID).

Mask Mode


The patient is positioned under normal fluoroscopic control to ensure that the region of anatomy under investigation is within the FOV of the image intensifier. A power injector is armed and readied to deliver 30 to 50 ml of contrast material at the rate of approximately 15 to 20 ml/s through a venous entry. If an arterial entry is utilized, 10 to 25 ml of diluted contrast material at 10 to 12 ml/s is typically used. The imaging apparatus is changed from the fluoroscopic mode o the DF mode. This requires an increase in x-ray tube current of 20 to 100 times the fluoroscopic mode and the activation of a program of pulse image acquisition. Mask mode results in successive subtraction images of contrast filled vessels. The injection is fired and after a delay of 4 to 10 s., before the bolus of contrast medium reaches the anatomic site, an initial x-ray pulsed exposure is made. The image obtained is stored in primary memory and displayed on the video monitor. This is the mask image. The mask image is followed by a series of additional images that are stored in adjacent memory locations, mask image is subtracted from each with the result stored in primary memory. At the same time the subtracted image is displayed on video monitor B. The digital subtraction of the static object allows a better analysis of the opacified arteries especially in their distal parts. The subtracted images appear in real time and are then stored in memory. After the exam, each subtracted image can be recalled for closer examination. Each image is obtained from a 33-ms x-ray pulse. The time required for one video frame is 33 ms. Because the video system is slow to respond and the video noise may be high, several video frames (usually 4-8 frames on average) may be summed in memory to make each image. This process is called image integration. Although this improves contrast resolution, it also increases patient dose because more image frames are acquired.

In mask-mode DF, the imaging sequence after acquisition of the mask can be manually controlled or pre-programmed. If pre-programmed, the computer controls the data acquisition according to the demands of the exam itself. If the computer capacity for acquiring images is sufficient, any combination of multiple delays and varying image acquisition rates is possible

Re-masking


If on subsequent examination the initial mask image is inadequate because of patient motion, improper technique or for some other reason, later images may be as the mask image.
A typical exam may require a total of 30 images in addition to the mask image.


Time-Interval Difference Mode


Some exams call for each subtracted image to be made from a different mask and follow-up frame.
In a cardiac study, for example, image acquisition begins 5 s after injection at the rate of 15 images per second for 4 s. A total of 60 images are obtained in such a study. These images are identified as frame numbers 1 through 60. Each image is stored in a separate memory address as it is acquired. If a TID of four images (268 ms) is selected, the first image to appear will be obtained when frame one is subtracted from frame five. The second image will contain the subtraction of frame two from frame six, the third will contain the subtraction of frame three from frame seven, and so on.

TID mode produces subtracted images from progressive masks and following frames.
In real time, the images observed convey the flow of the contrast medium dynamically.
Subsequent closer examination of each TID image shows it to be relatively free of motion artifacts but with less contrast than mask-mode imaging. As a result, TID imaging is principally applied in cardiac monitoring.

Misregistration


If patient motion occurs between the mask image and a subsequent image, the subtracted image will contain misregistration artifacts. The anatomy is not registered in the same pixel of the image matrix. This type of artifact can frequently be eliminated by reregistration of the mask, by shifting the mask by one or more pixels so that the superimposition of images is again obtained.



Energy Subtraction


Temporal subtraction techniques take advantage of changing contrast media during the time of the examination and require no special demands on the high-voltage generator.
Energy subtraction uses two different x-ray beams alternately to provide a subtraction image resulting from differences in photoelectric interaction. It is based on the abrupt change in photoelectric absorption at the K-edge of the contrast media compared with that of soft tissue and bone.This energy corresponds to the binding energy of the two K-shell electrons of iodine.
lWhen the incident x-ray energy is sufficient to overcome the K-shell electron binding energy of iodine, there is an abrupt and large increase in absorption. Graphically, this increase is known as the K absorption edge. Energy subtraction has a disadvantage of requiring some method of providing an alternating x-ray beam of two different emission spectra.
Two methods have been created, by alternately pulsing the x-ray beam at 70 kVp and then at 90 kVp, and introducing unlike metal filters into the beam alternately on a flywheel.




Hybrid Subtraction


Some DF systems are capable of combining temporal and energy subtraction techniques into what is called hybrid subtraction. Image acquisition follows the mask-mode procedure as described before. However, the mask and each subsequent image are formed by an energy subtraction technique. If patient motion can be controlled, hybrid imaging can theoretically produce the highest-quality DF images.


Patient Dose


One potential advantage to DF is reduced patient dose. DF images appear to be continuous, but in fact they are discrete. Most DF x-ray beams are pulsed to fill one or more 33-ms video frame, therefore, the fluoroscopic dose rate is lower than that for continuous analog fluoroscopy even though the mA setting may be higher. Static images with DF are also made with lower dose per frame than with 100-mm spot film camera. Both the television camera tube and a CCD have a higher sensitivity than the spot film. Digital spot images are so easy to acquire that it is possible to make more exposures than is necessary.



Picture Archiving and Communication System


Digital images come from nuclear medicine, digital ultrasound, digital subtraction angiography, CT, and MRI. Analog images, such as conventional radiographs, can be digitized using a various devices of today. Such film digitizers are based on laser beam technology. A picture archiving and communication system (PACS) when fully implemented, allows not only the acquisition but also the interpretation (soft copy) and storage of each medical image in digital form without resorting to film (hard copy). The projected efficiencies of time and cost are enormous.
The four principal components of a PACS are the image acquisition system, the display system, the network, and the storage system.

Display System


The heart of a PACS display system is either the cathode ray tube monitor of a video workstation or a solid-state flat panel display. To replace film viewing, each must be high resolution, at least 2048 X 2048. Current image matrices used with most digitally acquired images range from 256 X 256 to 1024 X 1024, which is considerably less than that required to equal the spatial resolution of film. PACS display stations are equipped with a keyboard and mouse control for the various image-processing modes. Some relaxation of the spatial resolution requirements of the workstation is allowed because of electronic image processing modes that are available. The simplest example is the ability to pan and zoom and change magnification of the displayed image. Image processing is possible because of the digital nature of the image and the interactive nature of the workstation.

Subtraction of one image from another emphasizes vascular structures.


Edge enhancement is effective for fractures and small, high-contrast objects.


Windowing is useful for amplifying soft tissue differences.


High-lightening can be effective in identifying diffuse nonfocal disease.


Pan, scroll, and zoom allow for careful visualization of precise regions of the image.


For these image-processing to be effective each workstation must be microprocessor-controlled and interact with each imaging device and the central computer. To provide this type of interaction to occur, a network is required.


Network


A network is used to describe the manner in which many computers can be connected to interact with one another. In radiology, in addition to general workstations, the network may consist of various types of images, PACS workstations, remote PACS workstations, a departmental mainframe, and a hospital mainframe. Clients are interconnected, usually by cable in a building, by telephone or cable television lines among buildings, and by microwave or satellite transmission to remote facilities.


The name teleradiology has been given to the process of remote transmission and viewing of images. There has been developed a standard imaging and interface format called DICOM, for Digital Imaging and Communications in Medicine. The network begins at the imaging system, where data are acquired in digital form. The images reconstructed from the data are processed at the console of the imaging system or transmitted to a PACS workstation for processing.
At any time, such images can be transferred to other clients within or outside the hospital.
Instead of running films up to surgery for viewing on a viewbox, one simply transfers the image electronically to the PACS workstation in surgery. When a radiologist is not immediately available for image interpretation, the image can be transferred to a PACS workstation in the radiologist’s home. Time is essential when considering image manipulation, and therefore fast computers and networks with high bandwidth are required for this task.
These requirements are relaxed for the information management and database portion of PACS, which is the Radiology Information System (RIS). From the RIS workstation, any number of coded diagnostic reports can be initiated and transferred to a secretarial workstation for report generation. The secretarial workstation in turn can communicate with the main hospital computer for patient identification, billing, accounting, and interaction with other departments.
These terminals can be utilized for scheduling of patients, technologists and radiologists and for analysis of departmental statistics. Finally, at the completion of an examination, PACS allows for more efficient image archiving.


Storage System


One motivation for PACS is archiving. This saves on films lost, spatial storage issues and film costs. With PACS, a film file room is replaced by a magnetic or optical memory device.
The future of PACS systems depends on the continuing development of the optical disk.
Optical disks can accommodate tens of GB of data and images and, when stored in a jukebox, can accommodate terabytes (TB).


END- Digital Fluoroscopy-Part III


Disclainer: "Capital Medical Equipment, Inc. and Sharon Moore will not be held liable for written materials within this blog or any future blogs. This blog was created as an educational tool and for no other purpose. It will be prohibitted by law to copy, reprint or publish this material without written permission of the author or Capital Medical Equipment, Inc."

Digital Fluoroscopy- Part II

Charge-Coupled Device (CCD) Camera






The charge-coupled device (CCD) was developed originally for a different reason. It originally replaced the television camera in the fluoroscopic chain.





The part that is sensitive in the CCD is a layer of crystalline silicon. When the silicon illuminates, an electrical charge is generated, which is then bundled into a sampling pixel by pixel, and than manipulated, to produce a digital image.






The CCD is mounted on the output phosphor of the image-intensifier tube and is coupled together by fiber optics or a lens system.






The main advantage of a CCD system is their small size and durability.




The spatial resolution of a CCD is determined by its physical size and pixel count.




Systems with a 1024 matrix (as mentioned in Digital Fluoroscopy Part I) can produce images with 10 lp/mm (line pairs/millimeter).




Television cameras show spatial distortion what is sometimes known as to “pin cushioning” or “barrel artifacts”.




There is no such distortion of these types with a CCD units.




The CCD has a higher sensitivity to light which is known as detective quantum efficiency (DQE) and has a lower level of electronic noise than you find on a television camera.




The sensitivity, or quantum efficiency, is simply the fraction of incident photons detected on the chip.




There is on average an 80% efficiency.




Because of this, there is higher signal-to-noise ratio (SNR) and better contrast resolution.





This results in lower patient exposure dose and the CCD response to light is consistent.





The CCD does not need to be warmed up and the image does not demonstrate a delay or lag in being seen.




The CCD unit itself has little or no maintenance.




The main feature of a CCD is it's response. In general, it has what is known as a linear response, which means as the exposure increases the response increases and as the exposure decreases the response decreases in a linear fashion.





Other image receptors have a sigmoid-shaped (S-shaped) response, which makes it difficult to image either very dim or very bright objects.






Information found on the graph in the high and low regions on a sigmoid shaped of response are lost.




The key point is with the linear response it makes it very useful for subtraction imaging (taking out only the important information from the image).




The result is improved dynamic range (images in motion) and better contrast resolution (shades of gray and blacks and whites are seen better).






END- Digital Fluoroscopy-Part II


Disclainer: "Capital Medical Equipment, Inc. and Sharon Moore will not be held liable for written materials within this blog or any future blogs. This blog was created as an educational tool and for no other purpose. It will be prohibitted by law to copy, reprint or publish this material without written permission of the author or Capital Medical Equipment, Inc."

Personal Introduction

My name is Sharon Moore. I have been a Radiologic Technologist for 35 years. I finished my preliminary career training in 1973. Upon completing that part of my education, I became a registered board certified radiologic technologist. In the next phase of my education, I completed a four year college program and received my Bachelor of Science degree in Radiologic Sciences. I have been involved in many facets of the Radiology profession through the course of 35 years and seen many changes to the health care industry.
My career has given me the opportunity to be involved in the role as a technologist with general radiology exams, emergency room procedures, operating room procedures, radiation therapy treatments and simulation, nuclear medicine studies, quality assurance, education and hospital administration. As my career progressed, I wore many hats including but not limited to: a technical coordinator in a very large hospital in Manhattan, a VP in a small community hospital, a clinical instructor, professor and clincial coordinator within hospital and college base educational programs in New Jersey. Now, I have joined forces with Barry Weinstein, President of Capital Medical Equipment, Inc. located in New Jersey. Many of you know Barry. I myself, have known him approximately 20 years. I met Barry through his father, Harvey. We remained in close contact through the years and now it brings us together in the buy/sell imaging equipment arena.

When looking back on my teaching, the areas that I most enjoyed were in radiation physics and equipment operations. I taught all aspects of radiation physics for radiography, ultrasound, nuclear medicine and radiation therapy. In addition, I taught about equipment operations and how physics are related. Based on my extensive experience in this field, I have developed a very strong understanding between physics and equipment.

I must point out that my knowledge is not name brand specific, but is applicable to all manufacturers. I am pleased to say that I will be posting blogs on a variety of topics to help educate our community. You can access this information anytime on our company's web site http://www.capmedonline.com/. Since I have a passion for teaching, I believe a forum setting would be a ideal place to educate and discuss equipment. So I have taken it upon myself to set up a blog for the purpose of educating. By reading the blog and presenting feedback, I hope the process will aid all of us with buying/selling our products.

My first topic which has many components for discussion, will be divided into a series of blogs on Digital Fluoroscopy. Part I has been posted and there will be a continuation of this until the topic has been completed. I will then continue to select a variety of topics that will help to educate our community.

Some of these topics you may be familiar with and some you may not. I encourage any readers to submit responses/comments if you wish. My goal is to help our community to be better prepared to handle basic equipment knowledge while serving the medical profession on day to day and long term projects. I hope that you find this educational and fun.

I look forward to hearing your feedback. Let's enjoy this ride together!

Digital Fluoroscopy- Part I

In this writing, I will try and give you an overview about Digital Fluorscopy. This is the first part of a series of blogs that will discuss many aspects of Digital Fluoroscopy and how it functions.

Introduction on Digital Fluoroscopy

The advantages of Digital Fluoroscopy (DF) over conventional fluoroscopy are the speed of image acquisition and post processing to enhance image contrast.

A matrix is described as how many pixels are used to create your image. Imagine that a pixel is a bucket of water, and now you need to fill a barrel which will represent your final image. Instead of looking at a digital image in volume such as a barrel of water, you are looking at an image that has a width and length to it. So, in creating an image, we will have so many pixels across for the width and so many pixels down for the length. The higher the number of pixels utilized in a matrix, the greater the amount of image storage information. Pixel arrangements have greatly increased through the years creating a more enhanced image. A very good example of this is our televisions in our homes. A standard tube like TV was designed to have a 525.5 (width) X 525.5 (length) lines of information to create our TV image we watch. Now today, we purchase high definition TV systems whose matrix arrangement is 1018 or higher, which would mean a 1018 X 1018 arrangement. Twice as much information stored in the same space yields much higher and better image quality. This premise is carried into the digital world when we look at pixel arrangements.

A 1024 x 1024 image matrix is sometimes called a 1000 line system.

In DF, the spatial resolution (how much information is stored within the space given) is determined by both the image matrix and by the size of the image intensifier.

•Spatial resolution is limited by the pixel size.
•DF Pixel size:
Pixel size = image intensifier size / matrix

Digital fluoroscopy is much the same as conventional fluoroscopy, where digital units have a computer added to it and usually two monitors instead of one monitor as with conventional fluoroscopy.
A DF examination is very similar to a conventional fluoroscopic study.
The console contains special function keys as well as alphanumeric keys, which are found for most equipment in the left module (side of key board), which allows patient data entry and information transfer ability to the computer.

The right portion of the console contains additional special function keys for data acquisition and image display.
It also contains computer interactive video controls and a pad for the cursor and region of interest (ROI) manipulation, where you can zoom in and magnify on specific anatomy.
Some systems may have joysticks or a mouse versus a pad.
Two monitors are used.
The left monitor is used to edit patient and examination data and to annotate final images.
The right monitor displays subtracted images (which are images, that look like our negatives from a role of film) by dulling background anatomy that is irrelevant and highlighting the relevant anatomy of interest.

High Voltage Generators
The fluoroscopic tube that lies beneath the table is a standard radiographic tube.
The tube current is measured in hundreds of mA (milliamperes or 1/1000 of an ampere) instead of less than 5 mA, as in a standard image-intensifier fluorscopy tube.

If the tube were energized continuously, it would fail because of thermal overloading and the patient dose would be exceedingly high.
Images from DF are obtained by pulsating the x-ray beam in a pattern known as pulse-progressive fluoroscopy.
During DF, the x-ray tube operates in the radiographic mode.
Image acquisition frame rates are 1 per second to 10 per second which are the most common ranges.
Because it requires 33 ms (millisecond or 1/1000 of a second) to produce one video frame, x-ray exposures longer than that can result in unnecessary patient dose.
The catch to this though is longer exposures reduce the noise levels (snow on image) and improve image quality.
X-ray generators must be capable of switching on and off very quickly.
The time that is required for the x-ray tube to be switched on and reach the selected level of kVp (kilovoltage peaks or 1000 volts) and mA is called the interrogation time.
The time required for the x-ray tube to be switched off is the extinction time.
DF systems have to incorporate three-phase or high-frequency generators with interrogation and extinction times of less than 1 ms.

END- PART I

In Part II of Digital Fluoroscopy we will continue this topic and examine the role of a CCD camera and how it is part of digital systems.

Disclainer: Capital Medical Equipment, Inc. and Sharon Moore will not be held liable for written materials within this blog or any future blogs. This blog was created as an educational tool and for no other purpose. It will be prohibitted by law to copy, reprint or publish this material without written permission of the author or Capital Medical Equipment, Inc."